1. Field of the Invention
The present invention relates to a method of fabricating a fine structure on a surface of an article to produce new optical, mechanical, or physical properties of the article. This method of fabricating a fine structure is applicable to fabrication of, for example, a MLA (micro-lens array), a diffractive optical element, a deflecting optical element, a refractive optical element, a complex refractive optical element, a fiber-related optical element, a beam splitter, and so on. Particularly, this method of fine structure fabrication is suitable for fabrication of an optical element having a dimension less than about 10 mm. Furthermore, this method is applicable to a surface treatment for a micro-machine, mechanically swingable parts of an automobile engine, a compressor of an air conditioner, and articles in various other fields.
2. Description of the Related Art
In the related art, quite a few techniques have been proposed for fabrication of a fine surface structure. These techniques are described below.    (A) The reference “Optical and Electro-optical Engineering Contact”, Vol. 38, No. 5, pp. 42–51 (2000) (particularly, descriptions on page 45) discloses a method of fabricating a multi-level element by lithography, which involves using a number of M masks and through a number of M−1 steps of treatments to produce an element having a step-like phase distribution having N=2M levels.    (B) The reference “Applied Physics”, Vol. 68, No. 6, pp. 633–638 (1999), published monthly by the Japan Society of Applied Physics, discloses a technique which combines methods of photolithography, dry etching, and direct writing using an electron beam, a laser beam, or an ion beam.    (C) Japanese Laid Open Patent Application No. 2001-356470 discloses a method involving fabricating a three-dimensional structure on a photo-conducting material by using a transmittance variable mask, and transferring the structure to the material of a final article by dry etching. Here, a transmittance variable mask is a mask that has a distribution of light transmittance in the thickness direction of the mask in addition to a pattern in the plane of the mask.    (D) Japanese Laid Open Patent Application No. 2002-192500 discloses a method in which a mold is fabricated in advance, and the surface shape of the mold is transferred to a resin, moreover the structure on the resin is transferred to the material of a final article by dry etching.
Among the methods above, method (A) requires a large number of masks, and non-negligible uncertainties produced by a large number of times of alignment. In addition, method (A) suffers from non-negligible etching errors in the thickness direction, and the minimum line width achievable with method (A) is 1 μm.
Method (B) requires an apparatus capable of high precision control, therefore, the apparatus is costly. In addition, the writing time using an electron beam, a laser beam, or an ion beam is long in method (B). As an numeral example, about 10 to 15 hours are needed to write a square area of 500 μm×500 μm. Because of poor mass-productivity and reproducibility of method (B), it has not been put into practical use.
In contrast, methods (C) and (D) are capable of producing three-dimensional surface structures with high precision and good reproducibility. Further, method (D) has a low fabrication cost because of usage of a mold for transfer of the surface structure.
However, there exists a difficulty in fabricating a fine surface structure on an object material by using dry etching as in method (D).
Considering the application of an article having a fine surface structure, the height of the surface structure is important. For example, in the case of an optical lens, a higher fine structure of the optical lens implies an increased numeral aperture and hence greater illuminance. But fabrication of a high surface structure is difficult because a large amount of the object material has to be etched. As a result, processing time (here, the etching time) becomes long, and fabrication precision degrades exponentially with the etching time.
In addition, when using a mold to fabricate a fine structure, because the mold is repeatedly used a large number of times, the service life of the mold is short.